Studying oily sludge treatment by thermo chemistry

2.1 Material

Oily sludge was obtained from the settlement tanks of the Second Oil Plant in Daqing of China. Setting tank is different from hydrocyclone, it relies on gravity to separate phases, and therefore it requires longer treatment time (Ijah and Ndana, 2003). In some cases, settlement does not occur even after extended periods because of insufficient gravity force for separating the oil and water and to remove the solid.

2.2 Methods

The gravimetric method was used to separate water, oil and solid in sludge samples, and then to estimate the moisture, hydrocarbon and total heavy metals in each sample. It should be noted that the gravimetric method is the method which is used to quantify the concentrations of oil and grease in soil, sediment, oily sludge, and other solid materials. A separate aliquot of as-received sludge sample, 5 g, was used for calculating the moisture content of oily sludge. The aliquot used for this determination was not used for further analysis. Moisture content was determined gravimetrically.

For the petroleum hydrocarbon analysis, another portion of the sludge (5 g) combusts in the muffle furnace at 600 °C, 5 h. And then the difference between the pre-and post-weight is calculated, so that oil content can be obtained.

In the certain quality (mass is M₁) of oily sludge, the per cent of oil is X. And a certain mass fraction of surfactant solution was put in a beaker, stirred for some time at a certain temperature, then the oil layer above was taken out after standing for a section of time. And the residua were put in Buchner funnel to filter. The filter paper used was dried and combusted to measure the residual oil content (residual oil content is S).

Residual oil rate is defined as follows: $$\text{residual oil rate}(\%)=\frac{S}{M_1\times X}$$ where X is the mass fraction of oil in oily sludge that had been measured by the gravimetric method.

3.1 Oil, water, and solid composition

Compositions of the sample are analyzed by using the gravimetric method. The results clearly indicate that oily sludge has a high percentage of hydrocarbons and it is 60.36%. The remaining is 29.26% moisture and just 10.38% solid. These percentages are very typical of oily sludge samples. Ideally, oily sludge will have a high concentration (>50%) of oil and a low solid content (<30%). The experiments were performed in duplicates to ensure the precision of values.

3.2 Influence of liquid/solid mass rate on residual oil rate

Other conditions were fixed, and only liquid/solid mass rate was changed at different solutions, the influence of liquid/solid mass rate for residual oil rate is studied. The results are shown in Fig. 1.

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Figure 1

Influence of liquid/solid mass rate on residual oil rate.

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From Fig. 1 the residual oil rates of samples which were treated by AEO-9, Peregal O, TritonX-100 and Na₂SiO₃·9H₂O gradually reduce with the increase of the liquid/solid mass rate. And the descending trends are more pronounced. The washing effects are gradually strengthening. The value at 5:1 is the lowest point and the washing effect is best. Residual oil rate of sample washed by DBS has no significant change and washing effect of DBS is inferior. AEO-9, Peregal O and TritonX-100 are nonionic surfactants. Nonionic surfactant is a compound of amphipathic structure that does not dissociate into ions in water is different from the ionic surfactant. They have high surface activity, their aqueous solutions have low surface tension and low critical micelle concentration (CMC). They can come up to critical micelle concentration at very low concentrations, so there will be the lowest point. And residual oil rate does not decline with the continuous increase of the concentration.

3.3 Influence of reaction temperature on residual oil rate

With the change of reaction temperature, the influence of reaction temperature for residual oil rate was studied. The results are shown in Fig. 2.

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Figure 2

Influence of reaction temperature on residual oil rate.

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Fig. 2 shows that the residual oil rates of samples used by Na₂SiO₃·9H₂O and Peregal O decreases with the rise of temperature, and the deoiling rate increases. Rise of temperature leads to the reduction of viscosity of crude oil and thermal expansion makes the adhesion ability of the oil film weakened. And because of the increase of temperature density difference between oil and water, the eluted oil floating as oil droplets and oil-water interface is clear and well layered. The washing effects of Peregal O, TritonX-100 and Na₂SiO₃·9H₂O are the best at 80°C. The residual rate of Nonionic surfactant TritonX-100 increases with temperature increasing, it slightly declines at 80 °C. And then increases again, this trend is related to nonionic surfactants cloud point, so the deoiling effect of TritonX-100 is best at 30 °C. From the point of the overall effect, the deoiling effect of AEO-9 is better than TritonX-100. And washing effect of DBS is not significant. The deoiling effect of DBS is less affected by temperature, the deoiling effect is best at 30 °C. When the temperature is below 40 °C, the deoiling effect of DBS is better than that of Na₂SiO₃·9H₂O. And when the temperature is higher than 40 °C, it is opposite.

3.4 Influence of reaction time on residual oil rate

The influence of reaction time for residual oil rate was studied with the change of reaction time. The results are shown in Fig. 3.

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Figure 3

Influence of reaction time on residual oil rate.

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From Fig. 3 deoiling rates increase with the increasing of reaction time. But deoiling rates reduce when reaction time sequentially increase. The extension of time leads to that oil-in-water (O/W) emulsion impedes further separation of oil and water. From Fig. 3 it is found that the optimum times of Peregal O and TritonX-100 are 30 min, and the optimum time of AEO-9 is 20 min. Through comparing experimental results the impact of reaction time on TritonX-100 is more obvious. The optimum time of DBS, AEO-9 and Na₂SiO₃·9H₂O are 20 min. When the reaction time is less than 20 min, the deoiling rates increase with the increasing of time. If the reaction time is more than 20 min, it is opposite. From the point of the overall effect, the deoiling effect of Na₂SiO₃·9H₂O is better than others.

3.5 Influence of eluent mass fraction on residual oil rate

The influence of eluent mass fraction for residual oil rate was studied with the change of eluent mass fraction. The results are shown in Fig. 4.

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Figure 4

Influence of eluent mass fraction on residual oil rate.

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From Fig. 4 it is shown that the optimum mass fraction of Peregal O and Na₂SiO₃·9H₂O is 3%. But the washing effect of Peregal O changes a lot, the curve of washing effect about Na₂SiO₃·9H₂O is smoother. When concentration of surfactant approaches to critical micelle concentration, micelles will appear in solution. And micelles increase with the rise of concentration, and the concentration of single surfactant molecules decreases. That gives rise to the decline of deoiling rate. From the point of the overall effect, the deoiling rate of TritonX-100 is superior to Peregal O. Because the concentration of AEO-9 approaches to its critical micelle concentration, the optimum mass fraction of AEO-9 is 2%. The optimum mass fraction of DBS is 5%, and its deoiling effect is obviously superior to Na₂SiO₃·9H₂O.

3.6 Comparison of experimental results

The optimal washing effects of every agent which was used to washing oily sludge and reclaim oil were compared, and the results are shown in Fig. 5.

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Figure 5

Solution washing effect comparison chart.

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Fig. 5 shows that these surfactants have a strong ability for decontaminating and washing oily sludge. All of these agents can make residual oil rate less than 3%. The washing effect of Na₂SiO₃·9H₂O is the best and the residual oil rate is only about 1.6%. That is because Na₂SiO₃·9H₂O has a very good dispersion effect.